1. Field of the Invention
The present invention relates to a digital photo-detector, and more specifically, to a thin film transistor array substrate for a digital photo-detector having reduced leakage current.
2. Discussion of the Related Art
An X-ray is a short wavelength radiation that easily passes through a subject, and transmittance of X-rays depends on a density of the subject. That is, inner characteristics of the subject may be indirectly observed through amount of X-rays passing through the subject.
An X-ray detector is a device that detects an amount of X-ray passing through the subject. The X-ray detector detects transmittance of X-ray and displays the inner characteristics of the subject on a display device. The X-ray detector may be generally used as a medical inspector, a non-destructive inspector, or the like.
In recent years, a digital X-ray detector using digital radiography (hereinafter referred to as a “DR”) without using a film is widely used as an X-ray detector. Each cell of a thin film transistor array for a digital X-ray detector includes a photo-diode (PIN diode) that receives X-rays, converts the X-rays into visible light, and converts the visible light into an electric signal. A thin film transistor that is formed under the photo-diode outputs the electric signal from the photodiode to a data line.
FIG. 1 is a view illustrating a configuration of a general digital X-ray detector 100. As shown in FIG. 1, the general digital X-ray detector 100 includes a thin film transistor array substrate 110, a bias supplier 120, a gate driver 130, a readout integrated circuit 150, a timing controller 180, and a power-supply voltage supplier 190. The readout integrated circuit 150 includes a signal detector 160 and a multiplexer 170.
The thin film transistor array substrate 110 detects an X-ray emitted from an energy source, converts the detected X-ray into an electric signal, and outputs an electric signal. The thin film transistor array substrate 110 includes a plurality of gate lines (GL), a plurality of data lines (DL) arranged in a vertical direction to the gate lines (GL) to define respective cell regions, and a plurality of photosensitive pixels (P) arranged in a matrix form in respective cell regions by the gate lines and the data lines.
Each photosensitive pixel (P) includes a photodiode (PD) that detects an X-ray and outputs a detection signal, e.g., photo-detection voltage, and at least one switching device for transmitting the detection signal output from the photodiode (PD) in response to a gate pulse. For example, the switching device is a transistor. Hereinafter, a configuration in which the switching device is a transistor will be described.
The photodiode (PD) senses an X-ray emitted from an energy source 10 and outputs the sensed signal as a detection signal. The photodiode (PD) is a device that converts incident light into an electrical detection signal through photoelectric effect and is, for example, a PIN diode a having structure including a p-type semiconductor layer, an intrinsic (I) semiconductor layer and an n-type semiconductor layer laminated in this order diode.
The bias supplier 120 applies a driving voltage through a plurality of bias lines (BL). The bias supplier 120 may apply a predetermined voltage to the photodiode (PD), or selectively apply a reverse bias or a forward bias thereto.
The gate driver 130 sequentially applies gate pulses having a gate-on voltage level through the gate lines (GL). In addition, the gate driver 130 may apply reset pulses having a gate-on voltage level to a plurality of reset lines (RL). The gate-on voltage level is a voltage level that turns on transistors of the photosensitive pixels (P). The transistors of the photosensitive pixels (P) may be turned on in response to the gate pulse or the reset pulse.
The detection signal output from the photodiode (PD) in response to the gate pulse is input through the data lines (DL) to the readout integrated circuit 150. The gate driver 130 may be mounted in an IC form at one side of the thin film transistor array substrate 110, or formed on a substrate, such as the thin film transistor array substrate 110, through a thin film process.
The readout integrated circuit 150 reads out the detection signal output from the turned-on transistor in response to the gate pulse. The readout integrated circuit 150 read outs a detection signal output from the photosensitive pixel P in an offset readout region to read out an offset image and an X-ray readout region to read out a detection signal after X-ray exposure.
The readout integrated circuit 150 may include a signal detector 160 and a multiplexer 170.
The signal detector 160 includes a plurality of amplification units that correspond to the data lines (DL) one to one and each amplification unit includes an amplifier (OP), a capacitor (CP), and a reset device (SW).
The timing controller 180 generates a start signal (STV), a clock signal (CPV) or the like and outputs the same to the gate driver 130 in order to control operation of the gate driver 130. In addition, the timing controller 180 generates a readout control signal (ROC), a readout clock signal (CLK) or the like and outputs the same to readout integrated circuit 150 in order to control operation of the readout integrated circuit 150. The gate driver 130 and the readout integrated circuit 150 may be operated using separate clock signals.
The power-supply voltage supplier 190 supplies a power-supply voltage to the photosensitive pixels (P) through the power-supply voltage lines (VDD).
A unit cell structure of the thin film transistor array for the X-ray detector will be described below.
FIG. 2 is a view illustrating a circuit configuration of a unit cell of a related art thin film transistor array substrate for a digital X-ray detector, FIG. 3 is a plan view illustrating the unit cell of the related art thin film transistor array substrate for a digital X-ray detector, and FIG. 4 is a sectional view taken along the line I-I′ of the unit cell of the related art thin film transistor array substrate for a digital X-ray detector.
The unit cell of the related art thin film transistor array substrate for a digital X-ray detector includes a plurality of gate lines (GL) to supply a scan signal, a plurality of data lines (DL) arranged in a direction vertical to the gate lines (GL) to output a data, a photodiode (e.g., a PIN-diode) formed in respective cell regions defined by the gate lines and the data lines to perform photoelectric conversion, a thin film transistor (TFT) formed at each of intersections between the gate lines (GL) and the data lines (DL) to turn on according to the scan signal of the gate lines and output the signal photo-electrically converted in the photodiode to the data lines, and a plurality of bias lines (BL) to apply a bias voltage to the photodiode. Here, a source electrode of the thin film transistor is connected to a first electrode of the photodiode in a lower part of the photodiode.
The cross-sectional structure of such a unit cell will be described below. As shown in FIG. 4, the gate line (represented by “GL” in FIGS. 2 and 3) and a gate electrode 2 protruding from the gate line are formed on a substrate 1, and a gate insulating film 3 is formed over the entire surface of the gate electrode 2.
In addition, an active layer 5 is formed on the gate insulating film 3 in a upper part of the gate electrode 2, and a drain electrode 4a and a source electrode 4b are formed at both sides of the active layer 5 to constitute the thin film transistor. A first interlayer insulating film 7 is formed over the entire surface of the substrate including the drain electrode 4a and the source electrode 4b, and the first interlayer insulating film 7 arranged on the source electrode 4b of the thin film transistor is selectively removed to form a first contact hole 6.
A first electrode 8 of the photodiode is formed on the first interlayer insulating film 7 such that it is connected to the source electrode 4b of the thin film transistor through the first contact hole 6. A semiconductor layer 9 having a p-type semiconductor layer, an intrinsic semiconductor layer and an n-type semiconductor layer is formed on the first electrode 8, and a second electrode 10 of the photodiode is formed on the semiconductor layer 9.
A second interlayer insulating film 11 is formed on the first interlayer insulating film 7 arranged over the entire surface of the substrate provided with the second electrode 10 of the photodiode, the first and second interlayer insulating films 7 and 11 arranged on the drain electrode 4a of the thin film transistor are selectively removed to form a second contact hole 16, and the second interlayer insulating film 11 arranged over the second electrode 10 of the photodiode is selectively removed to form a third contact hole 17.
A data line 12 (DL) connected through the second contact hole 16 to the drain electrode 4a of the thin film transistor is formed on the second interlayer insulating film 11. A light-shielding layer 13 is formed in a upper part of a channel region of the thin film transistor, and a bias line 14 (BL) connected through the third contact hole 17 to the second electrode 10 of the photodiode is formed. In addition, a protective film 15 is formed over the entire surface of the substrate.
As shown in FIGS. 3 and 4, the first contact hole 6 to connect the first electrode 8 for the photodiode to the source electrode 4b of the thin film transistor is formed under the photodiode.
The related art thin film transistor array substrate for a digital X-ray detector having this configuration operates as follows.
When an X-ray is irradiated, current flows in the photodiode according to amount of light corresponding to an intensity of the X-ray, and when a scan signal (a gate high voltage) is applied to the gate line, the thin film transistor turns on and outputs an optical signal through the data line.
However, the related art thin film transistor array substrate for a digital X-ray detector having this configuration has the following problems.
The general digital X-ray detector does not prevent current from flowing in the photodiode in a dark state, if it has an ideal structure. However, in the dark state, a dark current flows.
FIG. 5 is a graph showing leakage current measurement results when the numbers of contact holes are 1, 4, 16 and 36, and an inclination angle of contact holes is predetermined.
However, the thin film transistor array substrate for a digital X-ray detector has a low integration level that is not sufficient to reduce the inclination angle of the contact hole to a predetermined level. In addition, when exposure and etching processes for forming the contact hole are performed under partially different conditions, rather than under the same conditions, over the entire surface of the thin film transistor array substrate, leakage current characteristics are changed according to respective unit cells. Accordingly, reliability of the X-ray detector is deteriorated. In addition, characteristics of the photodiode are changed due to a source/drain electrode step of the thin film transistor, since the contact hole to connect the source electrode 4b of the thin film transistor to the first electrode 8 of the photodiode is formed in a lower part of the photodiode.